1. Field of the Invention
This invention concerns a method for correcting a B0 field drift in a temperature map exposure produced by means of magnetic resonance tomography. The invention furthermore concerns a device to implement such a method.
2. Description of the Prior Art
An image representation of an examination subject (in particular of a patient body) that graphically reproduces a spatial distribution of the temperature or of a temperature difference within the examination subject is designated as a temperature map. Such a temperature map can be acquired by magnetic resonance (MR) tomography by interaction of the patient with an MR scanner. The presentation of temperature maps that show the temporal change of the temperature distribution within a patient body is a central component of a successful implementation of a tumor ablation in interventional radiology. In order to kill the tumor tissue, the tumor to be removed is heated non-invasively by the application of focused ultrasound, or by the application of laser light or radio waves by means of a probe inserted into the patient body.
In order to be able to continuously monitor the temperature in the patient body during the intervention, the intervention typically occurs either directly in the MR scanner or in immediate proximity to the same. The temperature monitoring ensues by generating temperature maps of the patient body continuously or at regular intervals over a longer time span, typically of 30 to 120 minutes.
For the non-invasive temperature determination by means of magnetic resonance tomography, use is made of the physical effect that the nuclear magnetic resonance frequency of the hydrogen atoms (protons) in water molecules (in particular in the water molecules of the patient body) exhibits a characteristic temperature dependency. As a result of this temperature dependency, temperature differences in the patient body manifest themselves in a spatial variation of the phase of the magnetic resonance signal (represented in the form of complex numbers). Information about the temperature distribution in the patient body at the point in time of acquisition thus can be acquired from an image known as a phase image that reproduces this phase with spatial resolution. Phase images that were acquired at points in time to be compared are typically subtracted from one another on a pixel-by-pixel or voxel-by-voxel basis to determine the (spatially resolved) temperature difference in the patient body between different acquisition points in time. In interventional radiology, a “heated” phase image—i.e. a phase image acquired at a point in time during the intervention—is subtracted from an “unheated” phase image acquired before beginning the intervention. The temperature difference between the two acquisition points in time is then calculated for each pixel from the resulting difference image.
The precision of a temperature map generated in such a manner, in particular the precision with which the temperature difference between two acquisition points in time can be determined from such a temperature map at a specific location of the patient body, is typically impaired by a phenomenon known as the B0 field drift. This is a slow variation of the basic magnetic field (B0 field) of the MR scanner used to generate the temperature map.
A variation of the B0 field also disadvantageously leads to a change of the proton resonance frequency to which the acquisition of a temperature map is tied (in terms of measurement). Like temperature changes in the patient body, variations of the B0 field thus directly affect the phase images, so temperature-dependent phase changes and B0 field-dependent phase changes cannot be differentiated without further measures. An error in the resulting temperature map that normally grows with increasing distance between the acquisition points in time results due to the B0 drift.
The conventional approach to correct the influence of the B0 field drift in the temperature maps, is to select as correction regions spatial regions of the phase images to be compared in which no temperature change is expected between the acquisition points in time.
For B0 drift correction, the correction regions are calibrated to one another in the phase images to be compared. In other words, a phase image to be compared (normally the phase image acquired later) is corrected such that the correction region of the corrected phase image coincides in terms of the phase values with the correction region of the comparison image.
To achieve such a correction region, for the most part a magnetic resonance phantom filled with water that is placed in the image region of the phase image to be acquired has previously been used. Alternatively, a region of the patient body can also be defined as a correction region in which a heating (or also cooling) between the acquisition points in time can be precluded. The correction regions are in any case to be defined by the user.
Both methods for correcting the influence of the B0 field drift are relatively error-prone. In particular, a displacement of the phantom or—if the correction region was defined in the patient body—a change of the patient position between the acquisition points in time can then lead, as a result of then-incorrect B0 drift correction, to a large error of the resulting temperature map.
An error of the B0 drift correction can additionally result from the fact that nevertheless an unintended temperature change occurs in a correction region between the acquisition points in time. Such errors in particular occur when the correction region was selected in the patient body. The use of a phantom on the one hand often leads to a limited flexibility with regard to the slice selection and/or slice orientation, since the phantom must by necessity be contained in the selected image section.